The present invention relates generally to magnetic recording media and, in particular, to apparatus and techniques for testing the glide height characteristics of magnetic recording media.
Disc drives are the primary devices used for mass storage of computer programs and data. Within a disc drive, a load beam supports a hydrodynamic air bearing slider close to a rotating magnetic disc. The load beam supplies a downward force that counteracts the hydrodynamic lifting force developed by the slider's air bearing. During operation, the magnetic head rides at a distance from the surface of the magnetic disc. That distance must be small enough to allow high density recording while preventing damage that would otherwise be caused by contact between the spinning disc and the magnetic head.
High areal densities currently are achieved by reducing the separation between the disc and the head to less than twenty nanometers (nm). However, some level of disc roughness is required to reduce adhesive forces when the head is at rest. The level of disc surface topography must, therefore, be kept within a tight range to fly the head safely at low altitudes while simultaneously preventing it from sticking to the disc surface when the head is at rest. Thus, the topography of the disc surface is critical to the proper operation of the disc drive.
As part of the process of manufacturing hard files, the quality of a magnetic disc is provided by determining the glide conditions which can be established between the disc and a glide head. In particular, the effect of outwardly projecting defects on the surface of the magnetic disc is studied during glide height testing. When such defects are large enough to close the gap between the magnetic disc and the glide head, the defects strike the glide head. The movement of the glide head can be sensed, for example, by a sensor such as a piezoelectric transducer, which generates an electrical signal indicating the adjacent passage of an outwardly projecting defect.
During testing, a gliding action is brought about as a layer of air, dragged along by the spinning disc surface, is compressed between the surface of the disc and the adjacent surface of the glide head. As a result of the gliding action, the glide head rides at a distance from the surface of the disc. That distance is referred to as the "fly" height of the glide head and is determined, in part, by the peripheral speed of the rotating disc and the air pressure surrounding the disc. Thus, the fly height of the glide disc can be varied by changing the speed at which the disc rotates. A glide avalanche breaking point (GABP), which is used by engineers to characterize the surface of the disc, can be obtained based on the interaction between the disc surface and the glide head at different fly heights.
Several difficulties arise, however, when the linear velocity of the disc is varied to obtain a measure of the glide avalanche breaking point. The impact energy which is detected by a sensor depends on the velocity and, in some cases, is approximately proportional to the square of the velocity. Thus, it can be difficult to interpret the signals received by such a sensor. Furthermore, changing the linear velocity can affect the pitch and roll of the glide head. That, in turn, can affect the level of interference detected by the sensor. Additionally, the relationship between fly height and linear velocity may not be linear at very low speeds, such as speeds less than 200 inches per second, making it difficult to correlate the velocity with fly height.